Cellular Memory of HipA-Induced Growth Arrest: The Length of Cell Growth Arrest Becomes Shorter for Each Successive Induction.

Toxin-antitoxin (TA) systems are genetic modules found commonly in bacterial genomes. HipA is a toxin protein encoded from the hipBA TA system in the genome of Escherichia coli. Ectopic expression of hipA induces cell growth arrest. Unlike the cell growth arrest caused by other TA toxins, cells resume growth from the HipA-induced cell growth arrest phase after a defined period of time. In this article, we describe the change in the length of growth arrest while cells undergo repeated cycles of hipA induction, growth arrest and regrowth phases. In the multiple conditions tested, we observed that the length of growth arrest became successively shorter for each round of induction. We verified that this was not due to the appearance of HipA-resistant mutants. Additionally, we identified conditions, such as the growth phase of the starting culture and growth vessels, that alter the length of growth arrest. Our results showed that the length of HipA-induced growth arrest was dependent on environmental factors-in particular, the past growth environment of cells, such as a previous hipA induction. These effects lasted even after multiple rounds of cell divisions, indicating the presence of cellular "memory" that impacts cells' response to HipA-induced toxicity.

Comparative Proteomic Analysis of Propane Metabolism in Mycobacterium sp. Strain ENV421 and Rhodococcus sp. Strain ENV425.

While growing on propane as a sole source of carbon, many strains cometabolically degrade environmental pollutants, such as ethers and chlorinated hydrocarbons. To gain insights into the molecular basis behind such a high metabolic versatility of propanotrophs, we examined the propane-inducible protein expression patterns of 2 soil actinobacteria that are known to degrade a variety of ethers (i.e., Mycobacterium sp. strain ENV421 and Rhodococcus sp. strain ENV425). In both strains, soluble diiron monooxygenase(s), that would catalyze the first step of the pathway, were induced by propane. However, despite their phylogenetic similarity, different sets of additional putative propane oxygenases (e.g., cytochrome P450 and particulate methane monooxygenases) were overexpressed in the 2 strains. They also diverged in the expression of enzymes responsible for downstream reactions. This study revealed a diversity of expression of putative propane oxygenases, which may be responsible for xenobiotic degradation, as well as a variety of metabolic pathways for propane in these bacterial species.

Genomic Analysis of Propane Metabolism in Methyl Tert-Butyl Ether-Degrading Mycobacterium Sp. Strain ENV421.

Methyl tert-butyl ether (MTBE) is a ground water contaminant with plausible carcinogenic properties. Mycobacterium sp. strain ENV421 cometabolically degrades MTBE and other ethers during the growth on propane as a carbon source. In this study, the 6.2 Mb genome of strain ENV421 was deciphered. The genome sequence revealed the presence of numerous putative propane catabolic genes including genes encoding hydrocarbon oxygenases and short chain alcohol dehydrogenases. These data provide the basis for the elucidation of propane metabolic pathways in strain ENV421 and its application for the remediation of ground water contaminated with toxic ethers.